This is a single-center study of exercise testing in patients with sickle cell disease to look at lung function. The objective is to document the severity of pulmonary function abnormality in sickle cell patients and to correlate severity of pulmonary function abnormality with severity of underlying disease. The prediction is that patients with acute chest syndrome already have some degree of acute lung disease and through exercise testing, abnormalities will be found. The study is looking to see if acute chest syndrome is an antecedent to chronic lung disease.